Heat recovery from vacuum blowers on a paper machine

ABSTRACT

A papermaking process including the steps of forming a wet paper web on a papermaking machine, pulling vacuum through the web to remove water from the web and thereby generate discharge air, and diverting at least a portion of the discharge air through a control loop to a hot air drying system within the papermaking process to aid in drying the web, wherein the vacuum is generated by centrifugal blowers.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/684,848, filed Nov. 15, 2019 and entitled HEAT RECOVERY FROM VACUUMBLOWERS ON A PAPER MACHINE, which in turn claims priority to and thebenefit of U.S. Provisional Application No. 62/769,867, filed Nov. 20,2018 and entitled HEAT RECOVERY FROM VACUUM BLOWERS ON A PAPER MACHINE,the contents of which are incorporated herein by reference in theirentirety.

FIELD OF THE INVENTION

The present invention relates to a an improved process for making paper,and in particular is directed to a papermaking process that involvesapplication of vacuum to a wet web and hot air drying of the web.

BACKGROUND

Tissue manufacturers that can deliver the highest quality product at thelowest cost have a competitive advantage in the marketplace. A keycomponent in determining the cost and quality of a tissue product is themanufacturing process utilized to create the product. For tissueproducts, there are several manufacturing processes available includingconventional dry crepe, through air drying (“TAD”), or “hybrid”technologies such as Valmet's NTT and QRT processes, Georgia Pacific'sETAD, and Voith's ATMOS process. Each has distinctive differences inregards to installed capital cost, raw material utilization, energycost, production rates, and the ability to generate desired qualityattributes such as softness, strength, and absorbency. All of thesedistinctive differences need to be taken into account when choosing theproper process to make a tissue product.

The predominant manufacturing method for making a tissue web is theconventional dry crepe process. The process is the oldest tissuemanufacturing process and is thus well understood and optimized for highproduction rates. The major steps of the conventional dry crepe processinvolve stock preparation, forming, pressing, drying, creping,calendering (optional), and reeling the web.

The first step of stock preparation involves selection, blending,mixing, and preparation of the proper ratio of wood, plant, or syntheticfibers along with chemistry and fillers that are needed in the specifictissue grade. This mixture is diluted to over 99% water in order toallow for even fiber formation when deposited from the machine headboxinto the forming section. There are many types of forming sections usedin conventional papermaking (examples include inclined suction breastroll, twin wire C-wrap, twin wire S-wrap, suction forming roll, andCrescent formers) but all are designed to retain the fiber, chemical,and filler recipe while allowing the water to drain from the web.

After web formation and drainage (to around 35% solids) in the formingsection (assisted by centripetal force around the forming roll, andvacuum boxes in several former types), the web is transferred to a pressfabric upon which the web is pressed between a rubber or polyurethanecovered suction pressure roll and yankee dryer. The press fabric is apermeable fabric designed to uptake water from the web as it is pressedin the press section. It is composed of large monofilaments ormulti-filamentous yarns, needled with fine synthetic batt fibers to forma smooth surface for even web pressing against the yankee dryer.Removing water via pressing results in low energy consumption.

After pressing the sheet between a suction pressure roll and a steamheated cylinder (referred to as a yankee dryer), the web is dried fromup to 50% solids to up to 99% solids using the steam heated cylinder andhot air impingement from an air system (air cap or hood) installed overthe steam cylinder. The sheet is then creped from the steam cylinderusing a steel or ceramic doctor blade. This is a critical step in theconventional dry crepe process. The creping process greatly affectssoftness as the surface topography is dominated by the number andcoarseness of the crepe bars (finer crepe is much smoother than coarsecrepe). Some thickness and flexibility is also generated during thecreping process. If the process is a wet crepe process, the web must beconveyed between dryer fabrics through steam heated after-dryer cans todry the web to the required finished moisture content. After creping,the web is optionally calendered and reeled into a parent roll and readyfor the converting process.

The absorbency of a conventional tissue web is low due to the web beingpressed. This results in a low bulk, low void volume web where there islittle space for water to be absorbed. Additionally, bulk generated bycreping is lost when the web is wetted, which further reduces bulk andvoid volume.

The through air dried (TAD) process is another manufacturing method formaking a tissue web. The major steps of the through air dried processare stock preparation, forming, imprinting, thermal pre-drying, drying,creping, calendering (optional), and reeling the web. The stockpreparation and forming steps are similar to conventional dry creping.

Rather than pressing and compacting the web, as is performed inconventional dry crepe, the web undergoes the steps of imprinting andthermal pre-drying. Imprinting is a step in the process where the web istransferred from a forming fabric to a structured fabric (or imprintingfabric) and subsequently pulled into the structured fabric using vacuum(referred to as imprinting or molding). This step imprints the weavepattern (or knuckle pattern) of the structured fabric into the web. Thisimprinting step has a tremendous effect on the softness of the web, bothaffecting smoothness and the bulk structure. The design parameters ofthe structured fabric (including, for example, weave pattern, mesh,count, warp and weft monofilament diameters, caliper, air permeability,and optional over-laid polymer) are therefore critical to thedevelopment of web softness. The manufacturing method of an imprintingfabric is similar to a forming fabric (see U.S. Pat. Nos. 3,473,576,3,573,164, 3,905,863, 3,974,025, and 4,191,609 for examples) except forthe addition of an overlaid polymer. These types of fabrics aredisclosed in U.S. Pat. Nos. 5,679,222, 4,514,345, 5,334,289, 4,528,239and 4,637,859, for example. Essentially, fabrics produced using thesemethods result in a fabric with a patterned resin applied over a wovensubstrate. The benefit is that resulting patterns are not limited by awoven structure and can be created in any desired shape to enable ahigher level of control of the web structure and topography that dictateweb quality properties.

After imprinting, the web is thermally pre-dried by moving hot airthrough the web while it is conveyed on the structured fabric. Thermalpre-drying can be used to dry the web to over 90% solids before it istransferred to a steam heated cylinder. The web is then transferred fromthe structured fabric to the steam heated cylinder through a very lowintensity nip (up to 10 times less than a conventional press nip)between a solid pressure roll and the steam heated cylinder. The onlyportions of the web that are pressed between the pressure roll and thesteam cylinder rest on knuckles of the structured fabric; therebyprotecting most of the web from the light compaction that occurs in thisnip. The steam cylinder and an optional air cap system, for impinginghot air, then dry the sheet to up to 99% solids during the drying stagebefore creping occurs. The creping step of the process again onlyaffects the knuckle sections of the web that are in contact with thesteam cylinder surface. Due to only the knuckles of the web beingcreped, along with the dominant surface topography being generated bythe structured fabric, and the higher thickness of the TAD web, thecreping process has a much smaller effect on the overall softnessproperties compared to conventional dry crepe. After creping, the web isoptionally calendered and reeled into a parent roll and ready for theconverting process. Some TAD machines utilize fabrics (similar to dryerfabrics) to support the sheet from the crepe blade to the reel drum toaid in sheet stability and productivity. Patents which describe crepedthrough air dried products include, for example, U.S. Pat. Nos.3,994,771, 4,102,737, 4,529,480, and 5,510,002.

The TAD process is generally higher in capital costs than a conventionaltissue machine due to the amount of air handling equipment needed forthe TAD section, with higher energy consumption due to the need to burnnatural gas or other fuels for thermal pre-drying. The bulk softness andabsorbency is superior to conventional paper due to the superior bulkgeneration via structured fabrics which creates a low density, high voidvolume web that retains its bulk when wetted. The surface smoothness ofa TAD web can approach that of a conventional tissue web. Theproductivity of a TAD machine is less than that of a conventional tissuemachine due to the complexity of the process and especially thedifficulty in providing a robust and stable coating package on theyankee dryer needed for transfer and creping of a delicate pre-driedweb.

A variation of the TAD process where the sheet is not creped, but ratherdried to up to 99% using thermal drying and blown off the structuredfabric (using air) to be optionally calendered and reeled also exits.This process is called UCTAD or un-creped through air drying process. Anuncreped through air dried product is disclosed in U.S. Pat. No.5,607,551.

A new process/method and paper machine system for producing tissue hasbeen developed by the Voith company and is being marketed under the nameATMOS. The process/method and paper machine system has several patentedvariations, but all involve the use of a structured fabric inconjunction with a belt press. The major steps of the ATMOS process andits variations are stock preparation, forming, imprinting, pressing(using a belt press), creping, calendering (optional), and reeling theweb.

The stock preparation step is the same as that used in a conventional orTAD machine. The purpose is to prepare the proper recipe of fibers,chemical polymers, and additives that are necessary for the grade oftissue being produced, and diluting this slurry to allow for proper webformation when deposited out of the machine headbox (single, double, ortriple layered) to the forming surface. The forming process can utilizea twin wire former (as described in U.S. Pat. No. 7,744,726) a CrescentFormer with a suction Forming Roll (as described in U.S. Pat. No.6,821,391), or preferably a Crescent Former (as described in U.S. Pat.No. 7,387,706). The preferred former is provided with a slurry from theheadbox to a nip formed by a structured fabric (inner position/incontact with the forming roll) and forming fabric (outer position). Thefibers from the slurry are predominately collected in the valleys (orpockets, pillows) of the structured fabric and the web is dewateredthrough the forming fabric. This method for forming the web results in aunique bulk structure and surface topography as described in, forexample, U.S. Pat. No. 7,387,706 (FIG. 1 through FIG. 11 ). The fabricsseparate after the forming roll with the web staying in contact with thestructured fabric. At this stage, the web is already imprinted by thestructured fabric, but utilization of a vacuum box on the inside of thestructured fabric can facilitate further fiber penetration into thestructured fabric and a deeper imprint.

The web is now transported on the structured fabric to a belt press. Thebelt press can have multiple configurations. Belt press configurationsused in conjunction with a structured fabric can be viewed in U.S. Pat.No. 7,351,307 (FIG. 13 ), where the web is pressed against a dewateringfabric across a vacuum roll by an extended nip belt press. The pressdewaters the web while protecting the areas of the sheet within thestructured fabric valleys from compaction. Moisture is pressed out ofthe web, through the dewatering fabric, and into the vacuum roll. Thepress belt is permeable and allows for air to pass through the belt,web, and dewatering fabric, into the vacuum roll enhancing the moistureremoval. Since both the belt and dewatering fabric are permeable, a hotair hood can be placed inside of the belt press to further enhancemoisture removal as shown in FIG. 14 of U.S. Pat. No. 7,351,307.Alternately, the belt press can have a pressing device arranged withinthe belt which includes several press shoes, with individual actuatorsto control cross direction moisture profile, (see FIG. 28 in U.S. Pat.No. 7,951,269 or U.S. Pat. No. 8,118,979 or FIG. 20 of U.S. Pat. No.8,440,055) or a press roll (see FIG. 29 in U.S. Pat. No. 7,951,269 orU.S. Pat. No. 8,118,979 or FIG. 21 of U.S. Pat. No. 8,440,055). Thepreferred arrangement of the belt press has the web pressed against apermeable dewatering fabric across a vacuum roll by a permeable extendednip belt press. Inside the belt press is a hot air hood that includes asteam shower to enhance moisture removal. The hot air hood apparatusover the belt press can be made more energy efficient by reusing aportion of heated exhaust air from the yankee air cap or recirculating aportion of the exhaust air from the hot air apparatus itself (see U.S.Pat. No. 8,196,314). Further embodiments of the drying system composedof the hot air apparatus and steam shower in the belt press section aredescribed in U.S. Pat. Nos. 8,402,673, 8,435,384 and 8,544,184.

After the belt press is a second press to nip the web between thestructured fabric and dewatering felt by one hard and one soft roll. Thepress roll under the dewatering fabric can be supplied with vacuum tofurther assist water removal. This preferred belt press arrangement isdescribed in U.S. Pat. Nos. 8,382,956, and 8,580,083, with FIG. 1showing the arrangement. Rather than sending the web through a secondpress after the belt press, the web can travel through a boost dryer(FIG. 15 of U.S. Pat. Nos. 7,387,706 or 7,351,307), a high pressurethrough air dryer (FIG. 16 of U.S. Pat. Nos. 7,387,706 or 7,351,307), atwo pass high pressure through air dryer (FIG. 17 of U.S. Pat. Nos.7,387,706 or #7,351,307) or a vacuum box with a hot air supply hood(FIG. 2 of U.S. Pat. No. 7,476,293). U.S. Pat. Nos. 7,510,631,7,686,923, 7,931,781 8,075,739, and 8,092,652 further describe methodsand systems for using a belt press and structured fabric to make tissueproducts each having variations in fabric designs, nip pressures, dwelltimes, etc. and are mentioned here for reference. A wire turning rollcan be also be utilized with vacuum before the sheet is transferred to asteam heated cylinder via a pressure roll nip (see FIG. 2 a of U.S. Pat.No. 7,476,293).

The sheet is now transferred to a steam heated cylinder via a presselement. The press element can be a through drilled (bored) pressureroll (FIG. 8 of U.S. Pat. No. 8,303,773), a through drilled (bored) andblind drilled (blind bored) pressure roll (FIG. 9 of U.S. Pat. No.8,303,773), or a shoe press (see U.S. Pat. No. 7,905,989). After the webleaves this press element to the steam heated cylinder, the % solids arein the range of 40-50%. The steam heated cylinder is coated withchemistry to aid in sticking the sheet to the cylinder at the presselement nip and also aid in removal of the sheet at the doctor blade.The sheet is dried to up to 99% solids by the steam heated cylinder andinstalled hot air impingement hood over the cylinder. This dryingprocess, the coating of the cylinder with chemistry, and the removal ofthe web with doctoring is explained in U.S. Pat. Nos. 7,582,187 and7,905,989. The doctoring of the sheet off the yankee, creping, issimilar to that of TAD with only the knuckle sections of the web beingcreped. Thus the dominant surface topography is generated by thestructured fabric, with the creping process having a much smaller effecton overall softness as compared to conventional dry crepe. The web isnow calendered (optional) slit, and reeled and ready for the convertingprocess.

The ATMOS process has capital costs between that of a conventionaltissue machine and TAD machine. It has more fabrics and a complex dryingsystem compared to a conventional machine, but less equipment than a TADmachine. The energy costs are also between that of a conventional andTAD machine due to the energy efficient hot air hood and belt press.

The productivity of the ATMOS machine may be limited due to the abilityof the novel belt press and hood to dewater the web and web transfer tothe yankee dryer, likely driven by supported coating packages, theinability of the process to utilize structured fabric release chemistry,and the inability to utilize overlaid fabrics to increase web contactarea to the dryer. Adhesion of the web to the yankee dryer has resultedin creping and stretch development which may contribute to sheethandling issues in the reel section. The result is that the productionof an ATMOS machine may be below that of a conventional and TAD machine.

The bulk softness and absorbency of the ATMOS process is superior toconventional, but lower than a TAD web since some compaction of thesheet occurs within the belt press, especially areas of the web notprotected within the pockets of the fabric. Bulk may be limited sincethere is no speed differential to help drive the web into the structuredfabric as exists on a TAD machine.

The surface smoothness of an ATMOS web may be between that of a TAD weband conventional web. With use of an overlaid fabric, higher contactarea to the yankee dryer could be obtained resulting in finer crepe anda smoother surface.

The ATMOS manufacturing technique is often described as a hybridtechnology because it utilizes a structured fabric like the TAD process,but also utilizes energy efficient means to dewater the sheet like theConventional Dry Crepe process. Other manufacturing techniques whichemploy the use of a structured fabric along with an energy efficientdewatering process are the ETAD process and NTT process.

The ETAD process and products can be viewed in U.S. Pat. Nos. 7,339,378,7,442,278, and 7,494,563. This process can utilize any type of formersuch as a Twin Wire Former or Crescent Former. After formation andinitial drainage in the forming section, the web is transferred to apress fabric where it is conveyed across a suction vacuum roll for waterremoval, increasing web solids up to 25%. The web then travels into anip formed by a shoe press and backing/transfer roll for further waterremoval, increasing web solids up to 50%. At this nip, the web istransferred onto the transfer roll and then onto a structured fabric viaa nip formed by the transfer roll and a creping roll. At this transferpoint, speed differential can be utilized to facilitate fiberpenetration into the structured fabric and build web caliper. The webthen travels across a molding box to further enhance fiber penetrationif needed. The web is then transferred to a yankee dryer where it can beoptionally dried with a hot air impingement hood, creped, calendared,and reeled.

The ETAD process to date has been reported to have severe productivity,quality, and cost problems. Poor energy efficiency has been reported,bulk has been difficult to generate (likely due to high web dryness atthe point of transfer to the structured fabric), and softness has beenpoor (coarse fabrics have been utilized to generate target bulk, therebydecreasing surface smoothness). Absorbency is better than ATMOS due tothe ability to utilize speed differential to build higher bulk, but itis still below that of TAD which can create higher bulk with limited webcompaction that reduces void volume and thus absorbency. The installedcosts of an ETAD machine are unknown but likely close to that of a TADmachine due to the large amount of fabrics and necessary supportingequipment.

The NTT process and products can be viewed in international patentapplication publication WO 2009/067079 A1, US 2011/0180223 A1, and US2010/0065234 A1. The process has several embodiments, but the key stepis the pressing of the web in a nip formed between a structured fabricand press felt. The web contacting surface of the structured fabric is anon-woven material with a three dimensional structured surface comprisedof elevations and depressions of a predetermined size and depth. As theweb is passed through this nip, the web is formed into the depression ofthe structured fabric since the press fabric is flexible and will reachdown into all of the depressions during the pressing process. When thefelt reaches the bottom of the depression, hydraulic force is built upwhich forces water from the web and into the press felt. To limitcompaction of the web, the press rolls will have a long nip width whichcan be accomplished if one of the rolls is a shoe press. After pressing,the web travels with the structured fabric to a nip with the yankeedryer, where the sheet is optionally dried with a hot air impingementhood, creped, calendared, and reeled.

The NTT process has low capital costs, equal or slightly higher than aconventional tissue machine. It has high production rates (equal orslightly less than a conventional machine) due to the simplicity ofdesign, the high degree of dewatering of the web at the shoe press, andthe novelty of construction of the structured fabric. The structuredfabric, provides a smooth surface with high contact area to the dryerfor efficient web transfer. This high contact area and smooth surfacemakes the yankee coating package much easier to manage and createsconditions beneficial for fine creping, resulting in good sheet handlingin the reel section. The bulk softness of the NTT web is not equal tothe ATMOS sheet as the web is highly compacted inside the structuredfabric by the press felt compared to the ATMOS web. The surfacesmoothness is better than an ATMOS web due to the structured fabricdesign providing for better creping conditions. The NTT process alsodoes not have a speed differential into the structured fabric so thebulk and absorbency remains below the potential of the TAD and ETADprocesses.

The QRT process can be viewed in U.S. Patent Application Publication No.2008/0156450 A1 and U.S. Pat. No. 7,811,418. The process can utilize atwin wire former to form the web which is then transferred to a pressfabric or directly formed onto a press fabric using an inverted Crescentformer. The web can be dewatered across a suction turning roll in thepress section before being pressed in an extended nip between the pressfabric and a plain transfer belt. A rush transfer nip is utilized totransfer the web to a structured fabric in order to build bulk and moldthe web before the web is transferred to the yankee dryer and creped.This process alleviates the NTT design deficiency which lacks a rushtransfer or speed differential to force the web into the structuredfabric to build bulk. However, the costs, complexity, and likelyproductivity will be negatively affected.

As detailed, all of the above described tissue papermaking processesutilize vacuum somewhere in the process to remove water from the web.For example, in the conventional dry and wet creped tissue process,vacuum is utilized to remove water from the web using a suction pressureroll at the nip to the steam heated cylinder. In the TAD and UCTADprocesses, vacuum is utilized in the imprinting stage to pull the sheetinto the imprinting fabric as well as at various locations in theforming section to dewater the nascent web. The ATMOS process utilizesvacuum inside the vacuum roll across which the web inside thestructuring fabric is dewatertered between a press felt and belt press.Vacuum is also utilized on the ATMOS machine at the suction pressureroll that is nipped to the steam heated cylinder. In the ETAD process,after formation and initial drainage in the forming section, the web istransferred to a press fabric where it is conveyed across a suctionvacuum roll for water removal. The NTT process has several embodiments,but all utilize vacuum. For example, in FIG. 1 of U.S. Patent2010/0065234 A1, which is the typical machine configuration, a steam box(26) is utilized across a suction roll (25) to remove water from thenascent web prior to being imprinted by pressing between a press fabricand imprinting fabric in an extended nip press. In the QRT process, theweb can be dewatered across a suction turning roll in the press sectionbefore being pressed in an extended nip between the press fabric and aplain transfer belt. Additionally, each machine uses vacuum to removecontaminants from the various papermaking fabrics.

All the vacuum needs for the papermaking machine/process are centralizedusing vacuum pumps connected to a central or common piping header. Fromthis header, branch piping extends to the necessary points ofapplication and each branch can be controlled to a particular vacuumsetpoint using a control scheme which is typically a control loop usinga pressure indicating controller to measure the vacuum in the header andposition a control valve to maintain an operator inputted vacuumsetpoint for that header. There are two main types of vacuum pumps thatcan be utilized to generate the necessary vacuum level efficiency in alarge papermaking process; liquid ring vacuum pumps and centrifugalblowers.

Typically, the vacuum pump of choice for paper machine vacuum systemshave been liquid ring pumps due to efficiencies and the ability togenerate the very high vacuum capacities necessary in the papermakingprocess. Liquid ring vacuum pumps are a specific form of rotary positivedisplacement pump utilizing liquid as the principal element in gascompression. In the case of the liquid ring pumps used in thepapermaking processes, the liquid used is water and the gas beingcompressed is air. The compression is performed by a ring of liquidformed as a result of the relative eccentricity between the pumps'casing and a rotating multi-bladed impeller. The eccentricity results innear complete filling and then partial emptying of each rotor chamberduring every revolution. The filling and emptying action creates apiston action within each set of rotors of impeller blades. The pumps'components are positioned in such a manner as to admit gas when therotor chamber is emptying the liquid and then allowing the gas todischarge once compression is completed. The compression of gasgenerates significant heat through friction as the air molecules arebeing forced into closer contact. This heat is transferred into theliquid used in the vacuum pump. To prevent overheating, the liquid mustbe continually removed, cooled, and returned to the pump. Typically, theliquid is cooled using a cooling tower which simply transfers the wasteheat in the liquid to the atmosphere. Alternately, a liquid to liquid orliquid to air heat exchanger could be utilized to recover some of thethermal energy to be used elsewhere in the papermaking process.

Centrifugal blowers do not utilize liquid to compress the gas. The gasenters at the center of a set of spinning impellers, and is dividedbetween the impeller's vanes. As the impeller turns, it accelerates theair outwards using centrifugal force. This high-velocity air is thendiffused and slowed down in the surrounding blower housing to createvacuum. The compression of air through centripetal force can heat theair to excess of 160 deg C. This air is sometimes passed through an airto air or air to liquid heat exchanger to recover some of the thermalenergy and used elsewhere in the papermaking process. All the vacuumsystems described above contain separators to remove entrained water inthe air stream between the vacuum source (pump) and application point.

Although each of the papermaking processes provide advantages in webproduction and desired web characteristics, there is a continuing needfor more efficient papermaking processes.

SUMMARY OF THE INVENTION

As detailed above, each of the tissue papermaking processes also uses adrying step where hot air is impinged onto the nascent web or drawnthrough the nascent web to remove water. All the tissue papermakingprocesses except the UCTAD process utilize a hot air impingement systemto dry the web as it travels across a steam heated cylinder. The TAD andUCTAD processes utilize thermal drying by moving hot air through thenascent web as it travels on a structuring fabric across a hollowthrough air drying cylinder. The ATMOS process can utilize thermaldrying by moving hot air through the nascent web into a vacuum roll asit travels on a structuring fabric across the belt press.

An object of the present invention is to improve the efficiency of apapermaking process that uses vacuum to remove water from a paper weband hot air drying for drying of the web, by using at least a portion ofthe discharge air generated by the vacuum for the hot air drying of theweb.

The present invention provides improved processes for papermakingincluding: forming a wet paper web on a papermaking machine selectedfrom the group consisting of a through air drying machine and a hybridtissue machine; pulling vacuum through the web to remove water from theweb and thereby generate discharge air; and diverting a portion of thedischarge air to hot air drying areas within the papermaking process toaid in drying the web, wherein the vacuum is generated by centrifugalblowers.

A papermaking process according to an exemplary embodiment of thepresent invention comprises: forming a wet paper web on a papermakingmachine; pulling vacuum through the web to remove water from the web andthereby generate discharge air; and diverting at least a portion of thedischarge air through a control loop to a hot air drying system withinthe papermaking process to aid in drying the web, wherein the vacuum isgenerated by centrifugal blowers.

In an exemplary embodiment, the process further comprises the step ofcontrolling humidity of hot air within the hot air drying system.

In an exemplary embodiment, the step of controlling humidity comprises:detecting humidity of the hot air within the control loop; andcontrolling speed of an exhaust fan within the hot air drying systembased on the detected humidity so as to adjust the humidity of the hotair to a predetermined level.

In an exemplary embodiment, the step of controlling humidity furthercomprises controlling position of a fresh air damper within the controlloop based on the detected humidity.

In an exemplary embodiment, the process further comprises the step ofcontrolling pressure of the discharge air diverted to the hot air dryingsystem.

In an exemplary embodiment, the step of controlling pressure comprises:detecting pressure of the discharge air; and controlling position of adamper within the control loop based on the detected pressure so as toadjust the pressure of the discharge air to a predetermined level.

In an exemplary embodiment, the process further comprises the step ofcontrolling pressure of hot air within the hot air drying system.

In an exemplary embodiment, the step of controlling pressure of hot aircomprises: detecting pressure around a hot air impingement hood withinthe hot air drying system; controlling position of a vacuum exhaustdamper within the control loop based on the detected pressure so as toadjust the pressure of hot air to a predetermined level.

In an exemplary embodiment, the step of controlling pressure of hot airfurther comprises controlling speed of a discharge air supply fan basedon the detected pressure.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a control process according to anexemplary embodiment of the present invention; and

FIG. 2 is a schematic diagram of a control process according to anotherexemplary embodiment of the present invention.

DETAILED DESCRIPTION

The processes of the invention may result in significant energy savingsthat can be reaped by recovering the thermal energy in the discharge airstream created by the papermaking machine's vacuum system by using thehot discharge air as a direct source of make up air for the variousthermal hot air drying processes that have been discussed for drying anascent tissue web. This can be accomplished only when the vacuum systemuses blowers and not liquid ring vacuum pumps as will be explained.Thus, the various exemplary embodiments of the present invention areapplicable only to papermaking processes that involve the use of blowersas vacuum sources, and which do not include liquid ring vacuum pumps.

Because centrifugal vacuum pumps do not use liquid to compress air, thedischarge air stream is relatively dry and able to evaporate additionalwater making it suitable for use in hot air drying systems. Becauseliquid ring vacuum pumps utilize water to compress the air, thedischarge air is completely saturated and unable to evaporate anyadditional water thus making the air source unusable for any hot airdrying systems. Centrifugal vacuum pumps provide a relatively drydischarge air stream that is suitable for use in hot air drying systemson paper machines which actually have improved heat transfer withmoisture in the air up to 0.45 lb. of water per lb. of air.

To capture fully the waste heat from vacuum systems that use blowers,the discharged air from the blower could be directly used as makeup airto various thermal hot air drying processes that have been discussed fordrying a nascent tissue web, rather than recovering a portion of theenergy through a type of heat exchanger. All or a portion of thedischarge air may be sent to the thermal hot air dryers to comprisemakeup air. The amount of discharge air sent to the thermal hot airdryers may range from 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80% or 90% to100% of the discharge air. Conventional ducts may be attached to thepoint where vacuum air is discharged and run to the various hot airdryers forming a loop. Blowers may be utilized in line to facilitateflow of the discharge air to the hot air dryers. The loop may includemoisture sensors. If the moisture gets too high, the air flow isincreased. If the moisture gets too low, the air flow is decreased. Fastacting dampers may be included in the loop. Pressure sensors may also beincluded in the loop. If the pressure gets too high, the blowers may beshut down and/or the dampers may decrease the flow. The dampers may beopened or closed as needed, based on the pressure and moisture readings.

As previously explained, vacuum systems and hot air drying are used inall the tissue papermaking process. The discharge air from vacuumsystems that use centrifugal blowers can recover some waste heat fromthe discharge air using heat exchangers, but a near complete heatrecovery of the discharge air can be accomplished if the discharge airis used directly as a make-up air source for at least one of the hot airdrying processes used to dry the nascent web on any of the mentionedpapermaking machines/processes.

On any of the hot air drying systems, a fuel source is used to heat theair. Typically, the most economical fuel is natural gas. This heated airstream is pumped through the nascent web in one of the aforementioneddrying processes where it will remove water from the web. This airstream is recirculated with some portion of the air being removed tomaintain a level of humidity in the air that is optimal for heattransfer and thus removal of water from the nascent web. This level ofhumidity is roughly 0.45 lb. of water per lb. of dry air. Many factorsaffect the capacity and efficiency of a hot air stream to evaporate andremove water from a wet surface including temperature, pressure, andmoisture content of the air stream.

By using a humidity sensor and control loop, input and control totheoretically optimal humidity (moisture content) can be achieved forheat transfer based on thermodynamic calculations at varying temperatureand pressure conditions. The portion of air that is removed must bereplaced with fresh air that could have been passed through an air toair heat exchanger to reduce the natural gas needed to heat the air tothe necessary temperature setpoint. Rather than using fresh atmosphericair as the make-up source of air for the drying system, the inventivemethod uses the discharge air from the paper machine vacuum systems thatuse centrifugal blowers. The discharge temperature from a papermakingvacuum system using a centrifugal blower can be up to 160 deg C. and theflow rate of the discharge air may be up to 2800 cubic meters/min.

This hot, relatively dry source of air reduces the natural gas thatneeds to be utilized to reheat the recirculated air up to a normaloperating temperature of roughly 200 deg C. for most hot air dryingprocesses in the papermaking process.

The present invention also provides a control scheme that uses a twodamper control. The discharge air stream from the vacuum system to thehot air drying system may contain a pressure indicating controller toposition a fast acting damper or control valve to maintain an operatorinputted pressure setpoint. This setpoint will control the amount of airthat is used as makeup air in the hot air drying system along with thespeed control of the supply fan that recirculates the air through thesystem, and the exhaust fan that removes a portion of the wet air afterit has passed through and absorbed water from the nascent web. Theexhaust fan speed can be controlled by a humidity indicating controllerto maintain 0.451b water per lb. of air, while the supply fan speed canbe controlled by differential pressure indicating controllers that maycontrol the fan speed to maintain a differential pressure of air acrossthe nascent web. A fast acting damper for purging to atmosphere may belocated on the vacuum discharge air line during startup and shutdownconditions of the hot air heating system.

The vacuum blowers may be shut down before the TAD blowers. In the eventthat pressure builds up in the system, a rupture disc or weight liftdoor may be included in the loop. The rupture disc or weight lift doormay be positioned before the TAD recirculation loop, but after exhaustof the blower.

The invention illustratively disclosed herein suitably may be practicedin the absence of any component, ingredient, or step which is notspecifically disclosed herein. Several examples are set forth below tofurther illustrate the nature of the invention and the manner ofcarrying it out. However, the invention should not be considered asbeing limited to the details thereof. All patents discussed above arehereby incorporated by reference.

FIG. 1 shows a control scheme according to an exemplary embodiment ofthe present invention for utilization of the exhaust air from a papermachine centrifugal blower vacuum pumps into a paper machine Through AirDrying system. All the vacuum needs for the papermaking machine/processare centralized using vacuum pumps connected to a central or commonpiping header (1). From this header, branch piping (2) extends to thenecessary points of application (3) and each branch can be controlled toa particular vacuum set-point using a control scheme which is typicallya control loop using a pressure transmitter (4) and pressure indicatingcontroller (5) to measure the vacuum in the branch and position acontrol valve (6) to maintain an operator inputted vacuum set-point forthat branch. The vacuum header contains a separator (7) to removeentrained water in the air stream between the vacuum source (vacuumblower 8) and application point. Water removed from the separator can bereturned to the paper machine white water system (9).

The discharge air stream header leaving from the vacuum system can splitinto multiple branch lines (for example, two branch lines (100), (200)as shown in FIG. 1 ) to feed more than one hot air drying system.Although only branch line (200) is shown in FIG. 1 , it should beappreciated that second branch line (100) will have the same controlsand feed duplicate hot air drying systems. Each branch line to its hotair drying system may contain a pressure transmitter (10) and pressureindicating controller (11) to position a fast acting damper (12) orcontrol valve (12) to maintain an operator inputted pressure set-point.This set-point will control the amount of pressure in the discharge airstream branch line by relieving pressure to atmosphere. The pressureset-point is recommended to be from 0.1 to 0.5 kpa in order to maintainslight pressure in the line as to not cause back pressure on the vacuumblower (8). Fast acting dampers (13 and 14) are used for purging toatmosphere during startup and shutdown conditions of the hot air dryingsystem. The damper (13) on the discharge air stream would close for thepurging and startup and shutdown conditions, while the second fastacting damper (14) would open to allow fresh air (15) into the airsystem to purge any remaining natural gas before firing the system's gasburner (16). During normal operating conditions, the fresh air damper(14) and vacuum exhaust damper (13) are controlled by an inputtedset-point by the operator.

The vacuum system discharge air stream then enters the through airdrying, hot air system upstream of the supply fan (17). The speed of thesupply fan is controlled by an operator inputted set-point. The air isheated in the combustion chamber (16) before entering a hood (18) whichextends over the paper web path (19) where the paper web is conveyed ona structuring fabric across a through air drying cylinder or drum (20).Along the perimeter of the hood on both the front and back side of thehood, there are located a series of pressure transmitters (21). Thesetransmitters measure the pressure of air at the interface of the hoodand drying cylinder along the perimeter where atmospheric air could bepulled through the gap that exists at the interface of the hood anddrying cylinder. To ensure no cold atmospheric air is pulled through thegap into the air system, these transmitters relay the pressure data to apressure indicating controller (22) which controls the speed of anexhaust fan (23) to maintain a pressure set-point entered by theoperator. Typically, the pressure indicating controller will take theaverage reading of the pressure transmitters. The pressure set-point isgenerally 0.1 kpa which allows a slight amount of pressure and thus airto escape from the gap between the hood and the cylinder to prevent anycold air from atmosphere from entering the hot air system. After the hotair passes through the paper web and into the hollow cylinder, the airstream is recirculated back to the supply fan (17). A weighted lift door(25) may be installed on the vacuum discharge air stream piping betweenthe two fast acting dampers. The purpose of this device is to safelyrelieve pressure in the event of failure of the fast acting dampers tocontrol or relieve pressure in the line. The weighted lift door willlift or open prior to potential failure of the piping due toover-pressurization. It will also close after relieving the excesspressure. The weighted lift door is located on the piping in a locationwhere any discharge air is relieved to atmosphere away from any possiblepersonnel.

EXAMPLE 1

Tissue paper was made on a wet-laid asset comprised of a twin wire gapforming section containing a three layer headbox, a predrying sectioncontaining two through air drying drums, a drying section containing aYankee steam cylinder with a hot air impringment hood, a reel belt forwinding the paper towel web onto a spool, and a vacuum system containingvacuum blowers where the discharge air can be utilized as makeup air forthe through air drying drums as explained above.

The tissue web was multilayered with the fiber and chemistry of eachlayer selected and prepared individually to maximize product qualityattributes of softness and strength. The first exterior layer, which wasthe layer that contacted the Yankee dryer, was prepared using 80%eucalyptus with 0.25 kg/ton of the amphoteric starch and 0.25 kg/ton ofthe glyoxylated polyacrylamide. The interior layer was composed of 40%northern bleached softwood kraft fibers, 60% eucalyptus fibers, and 1.0kg/ton of T526 a softener/debonder (EKA Chemicals Inc., 1775 West OakCommons Court, Marietta, GA, 30062). The second exterior layer wascomposed of 20% northern bleached softwood kraft fibers, 80% eucalyptusfibers and 3.0 kg/ton of Redibond 2038. Softwood was refined at 115kwh/ton to impart the necessary tensile strength.

The fiber and chemicals mixtures were diluted to a solids of 0.5%consistency and fed to separate fan pumps which delivered the slurry toa triple layered headbox. The headbox pH was controlled to 7.0 byaddition of a caustic to the thick stock before the fan pumps. Theheadbox deposited the slurry to a nip formed by a forming roll, an outerforming wire, and inner forming wire. The slurry was drained through theouter wire, which was a KT194-P design supplied by Asten Johnson (4399Corporate Rd, Charleston, SC (843) 747-7800)), to aid with drainage,fiber support, and web formation. When the fabrics separated, the webfollowed the inner forming wire and was dried to approximately 25%solids using a series of 4 vacuum boxes and a steam box. The fabricswere running at 1450 meters/min. While this Example utilizes 4 vacuumboxes in the forming section, the system may utilize 1, 2, 3, 4, 5, 6, 7or 8 vacuum boxes in the forming section.

The web was then transferred to a structured fabric with the aid of avacuum box to facilitate fiber penetration into the structured fabric.The structured fabric was a Prolux 005 design supplied by Albany (216Airport Drive Rochester, NH 03867 USA) and was a 5 shed design with awarp pick sequence of 1, 3, 5, 2, 4, a 17.8 by 11.1 yarn/cm Mesh andCount, a 0.35 mm warp monofilament, a 0.50 mm weft monofilament, a 1.02mm caliper, with a 640 cfm and a knuckle surface that was sanded toimpart 27% contact area with the Yankee dryer. The web was dried withthe aid of two TAD hot air impingement drums to 85% consistency beforetransfer to the Yankee dryer.

The web was held in intimate contact with the Yankee surface using anadhesive coating chemistry. The Yankee was provided steam at 3.0 barwhile the installed hot air impingement hood over the Yankee blew heatedair up to 450 deg C. The web was creped at 97.5% consistency from theYankee at 10% crepe using a ceramic blade at a pocket angle of 90degrees. The web was cut into two of equal widths using a high pressurewater stream at 10,000 psi and reeled into two equally sized parentrolls and transported to the converting process.

The vacuum system containing blowers (8) exhausted air into a header ata volumetric flow rate of 5000 m∧3/min at 170 deg C. This headerbranches (100, 200) to duplicate air handling systems of both the firstand second TAD drum. The exhaust air from the vacuum blowers wasdischarged to the atmosphere by closing damper (13) to 0% and openingdamper (12) to 100% on the distributed control system (DCS) on each TADdrum air handling system.

The air handling system for the first TAD drum contains a supply fan,exhaust fan, and combustion fan. The combustion fan provides air tocombust natural gas inside the combustion chamber (16). 1,200 normalcubic meters per hour of natural gas was combusted to heat therecirculaing air in the first TAD drum air handling system to 180 deg C.The supply fan (17) is capable of recirculating 11,600 m∧3 of air perminute. This fan was running at 80% speed using a variable speed driveset by the operator in the DCS. This air is moved through the supply fanand impinged through the paper web and into the first TAD drum (20).

After passing through the paper web and into the TAD drum, thetemperature of the air has been reduced to 104 deg C. as it evaporateswater from the web. An exhaust fan (23) ran at 72% speed, set in the DCSby the operator. This fan has maximum air handling capacity of 2500m∧3/min and is used to remove a portion of the humid air from thesystem. The remainder of the air was returned to the combustion chamberand recirculated through the process. Fresh make up air was provided byopening the fresh air damper (14) to 30% on the DCS.

The web then passed across a second TAD drum for further water removalfrom the paper web. The air handling system for the second TAD drum wasidentical to the first TAD drum.

As previously stated, the fast action damper (12) is open to 100% andfast acting damper (13) is closed to 0% on the DCS on the second TAD airhandling system, same as on the first TAD air handling system.

The combustion chamber (16) utilized 275 normal cubic meters per hour ofnatural gas to heat the recirculaing air in the second TAD drum airhandling system to 105 deg C. The supply fan (17) is capable ofrecirculating 11,600 m∧3 of air per minute. This fan was run at 65%speed using a variable speed drive set by the operator in the DCS. Thisair is moved through the supply fan and impinged through the paper weband into the second TAD drum (not shown). After passing through thepaper web and into TAD drum, the temperature of the air had been reducedto 86 deg C. An exhaust fan (23) ran at 60% speed set in the DCS by theoperator. This fan has maximum air handling capacity of 2500 m∧3/min andis used to remove a portion of the humid air from the system. Theremainder of the air was returned to the combustion chamber andrecirculated through the process. Fresh make up air was provided byopening the fresh air damper (13) to 15% on the DCS. The paper web leftthe second TAD drum at 85% consistency before being dried in the dryingsection to 97.5% consistency prior to being reeled onto a spool.

Next, the vacuum system containing blowers (8) was utilized with theexhaust air from the vacuum blowers being discharged as makeup air tothe first and second TAD air handling systems. This was accomplished byopening fast action damper (13) to 100% and by closing fast actingdampers (12) and (14) on each air handling system on the DCS. Then thepressure setpoint on the vacuum blower discharge air header wascontrolled to a setpoint of 0.1 kpa by using pressure controllers (11)to modulate fast action damper (12) on each air system. All remainingdamper and fan speed setpoints remained unchanged with the target of 85%consistency leaving the second TAD drum remaining. The natural gas usedropped from 1200 to 900 normal cubic meters per hour in the first TADcombustion chamber. The natural gas use dropped from 275 to 50 normalcubic meters per hour in the second TAD combustion chamber.

FIG. 2 shows a control scheme according to another exemplary embodimentof the present invention for utilization of the exhaust air from papermachine centrifugal blower vacuum pumps into the paper machine ThroughAir Drying system. All the vacuum needs for the papermakingmachine/process are centralized using vacuum pumps connected to acentral or common piping header (51). From this header, branch piping(52) extends to the necessary points of application (53) and each branchcan be controlled to a particular vacuum set-point using a controlscheme which is typically a control loop using a pressure transmitter(54) and pressure indicating controller (55) to measure the vacuum inthe branch and position a control valve (56) to maintain an operatorinputted vacuum set-point for that branch. The vacuum header containsseparators (57) to remove entrained water in the air stream between thevacuum source (vacuum blower 58) and application point. Water removedfrom the separator can be returned to the paper machine white watersystem (59).

The discharge air stream header leaving from the vacuum system can splitinto multiple branch lines (for example, two branch lines (1000), (2000)as shown in FIG. 2 ) to feed more than one hot air drying system.Although only branch line (2000) is shown in FIG. 2 , it should beappreciated that second branch line (1000) will have the same controlsand feed duplicate hot air drying systems. Each branch line to its hotair drying system may contain a pressure transmitter (60) and pressureindicating controller (61) to position a fast acting damper (62) orcontrol valve (62) to maintain an operator inputted pressure set-point.This set-point will control the amount of pressure in the branch line byrelieving pressure to atmosphere. The pressure set-point is recommendedto be from 0.1 to 0.5 kpa in order to maintain slight pressure in theline as to not cause back pressure on the vacuum blower (58).

The discharge air stream from the vacuum system would then pass througha fast acting damper (63) which is the primary device to control the hotair system pressure along with the secondary device of the supply fan(67). These two devices are controlled by a pressure indicatingcontroller (72). The pressure indicating controller receives pressurereadings from pressure transmitters (71) installed along the perimeterof the hood on both the front and back side of the hood. Thesetransmitters measure the pressure of air at the interface of the hoodand drying cylinder along the perimeter where atmospheric air can bepulled through the gap that exists at the interface of the hood anddrying cylinder. Typically, the pressure indicating controller will takethe average reading of the pressure transmitters. The pressure set-pointis operator inputted and is recommended at 0.1 kpa, which allows aslight amount of pressure and thus air to escape from the gap betweenthe hood and the cylinder to prevent any cold air from atmosphere fromentering the hot air system. This control loop is configured to maximizethe position of the fast acting damper (63) while minimizing the speedof the supply fan (67) to maintain the pressure set point. This allowsmaximum utilization of the heat and kinetic energy that has already beenexpended into the vacuum discharge air stream before expending moreenergy at the supply fan. Limits to the range of motion to the fastacting damper (63) and supply fan speed (67) can be used in the controlloop as well as how quickly these devices should adjust to maintain thepressure set-point.

The vacuum system discharge air stream then enters the through air driedhot air system upstream of the supply fan (67). The air is heated in thecombustion chamber (66) before entering a hood (68) which extends overthe paper web path (69) where the paper web is conveyed on a structuringfabric across a through air drying cylinder or drum (70).

After the hot air passes through the paper web and into the hollowcylinder, the air stream is recirculated back to the supply fan (67). Asthe air stream travels to the supply fan the humidity is measured by ahumidity indicating transmitter (74) using a humidity indicatingcontroller (75) to control the speed of the primary control device,exhaust fan (73) and the secondary control device, fresh air fast actingdamper (64). The humidity set-point is operator inputted and recommendedat 0.45 pounds of water per pound of air for maximum heat transfer andthus drying efficiency. This control loop is configured to maximize thespeed of the exhaust fan 73) and minimize the position of the fastacting damper (64) to maintain the humidity set-point. This allowsmaximum utilization of the heat and kinetic energy that has already beenexpended into the hot air drying system before allowing cold atmosphericair (65) to enter through the fast acting damper, which requires moreenergy to heat than the energy required at the exhaust fan. Limits tothe range of motion to the fast acting damper (64) and speed of theexhaust fan (73) can be used in the control loop as well as how quicklythese devices should adjust to maintain the humidity set-point.

Fast acting dampers (63 and 64) are also used for purging to atmosphereduring startup and shutdown conditions of the hot air heating system.The damper (63) on the discharge air stream would close for the purgingand startup and shutdown conditions, while the second fast acting damper(64) would open to allow fresh air (65) into the air system to purge anyremaining natural gas before firing the systems' gas burner (66). Aweighted lift door (76) may be installed on the vacuum discharge airstream piping between the two fast acting dampers. The purpose of thisdevice is to safely relieve pressure in the event of failure of the fastacting dampers to control or relieve pressure in the line. The weightedlift door will lift or open prior to potential failure of the piping dueto over-pressurization. It will also close after relieving the excesspressure. The weighted lift door is located on the piping in a locationwhere any discharge air is relieved to atmosphere away from any possiblepersonnel.

EXAMPLE 2

Referring back to the final setpoints achieved in Example 1, the controlschemes outlined in Example 2 are followed. The pressure on the vacuumblower discharge air header was controlled to a setpoint of 0.1 kpa byusing pressure controllers (61) to modulate fast action dampers (62) oneach duplicate air system. Next, the temperature controls of both TAD1and TAD 2 air systems are placed into automatic control in the DCS. OnTAD 1, the exhaust temperature setpoint of 104 deg C. is input into theDCS and the temperature setpoint of the air leaving the combustionchamber (66) is automatically modulated to maintain the exhaust airtemperature at 104 deg C. On TAD #2, the temperature setpoint of the airleaving the combustion chamber (66) is controlled to maintain aconsistency setpoint in the paper web of 85% as measured on the webimmediately prior to transfer to the yankee dryer. The device used tomeasure consistency measures the average consistency of the web acrossthe entire width of the web. Next, the pressure indicating controller(72) on each TAD air handling system is placed into DCS control with asetpoint of 0.1 kpa which modulates the speed of the supply fans (67)and position of the fast acting damper (63) to maintain the pressuresetpoint. Finally, the humidity controllers on each TAD air handlingsystem are placed into DCS control where the exhaust fans (73) and freshair fast acting dampers (64) are modulated to maintain the humidityratio of the exhaust air stream at 0.45 lbs of water per lb of air.While the humidity ratio of 0.45 lbs of water per lb of air is used inthis example, ratios of 0.30 to 0.60 lbs of water per pound of air maybe utilized. With these controls now implemented, the following changeswere observed.

On TAD 1 air handling system, fast acting damper (62) modulated between0-5% open to maintain the vacuum discharge air header pressure at 0.1kpa. The humidity controller increased the speed of the exhaust fan from72% to 93% while keeping fast acting damper (64) closed. The pressureindicating controller (72) on TAD 1 increased the supply fan speed from80% to 91% while keeping fast acting damper (63) at near 100% open. Thenatural gas flow decreased from 900 to 810 normal cubic meters per hourof natural gas. On TAD 2 air handling system, fast acting damper (62)modulated between 0-5% open to maintain the vacuum discharge air headerpressure at 0.1 kpa. The humidity controller increased the speed of theexhaust fan from 60% to 73% while keeping fast acting damper (64)closed. The pressure indicating controller (72) on TAD #2 increased thesupply fan speed from 65% to 71% while keeping fast acting damper (63)at near 100% open. The natural gas air flow decreased from 50.7 to 23.5normal cubic meters per hour of natural gas.

Now that the preferred embodiments of the present invention have beenshown and described in detail, various modifications and improvementsthereon will become readily available to those skilled in the art.Accordingly, the spirt and scope of the present invention is to beconstrued broadly and not limited by the foregoing specification.

We claim:
 1. A papermaking machine comprising: one or more centrifugalblowers that pull vacuum through a wet paper web at at least one webpre-drying location within the papermaking machine where water isremoved from the web and discharge air is generated; and a control loopthat diverts at least a portion of the discharge air to a hot air dryingsystem disposed at a final web drying location within the papermakingmachine to aid in drying the web.
 2. The papermaking machine of claim 1,further comprising a humidity controller configured to control humidityof hot air within the hot air drying system.
 3. The papermaking machineof claim 2 further, wherein the humidity controller is configured tocontrol humidity of the hot air to a predetermined level by controllingspeed of an exhaust fan within the hot air drying system based ondetected humidity within the control loop.
 4. The papermaking machine ofclaim 3, wherein the humidity controller is further configured tocontrol humidity of the hot air by controlling position of a fresh airdamper within the control loop based on the detected humidity.
 5. Thepapermaking machine of claim 1, further comprising a discharge airpressure controller configured to control pressure of the discharge airdiverted to the hot air drying system.
 6. The papermaking machine ofclaim 5, wherein the discharge air pressure controller is configured tocontrol pressure of the discharge air to a predetermined level bycontrolling position of a damper within the control loop based ondetected pressure of the discharge air.
 7. The papermaking machine ofclaim 1, further comprising a hot air pressure controller configured tocontrol pressure of hot air within the hot air drying system.
 8. Thepapermaking machine of claim 7, wherein the hot air pressure controlleris configured to control pressure of hot air within the hot air dryingsystem to a predetermined level by controlling position of a vacuumexhaust damper within the control loop based on detected pressure arounda hot air impingement hood within the hot air drying system.
 9. Thepapermaking machine of claim 8, wherein the hot air pressure controlleris further configured to control pressure of hot air within the hot airdrying system by controlling speed of a discharge air supply fan. 10.The papermaking machine of claim 1, wherein the papermaking machine is aThrough Air Dried (TAD) papermaking machine.
 11. The papermaking machineof claim 1, wherein the papermaking machine is an Advanced TissueMolding System (ATMOS) papermaking machine.
 12. The papermaking machineof claim 1, wherein the papermaking machine is a New Tissue Technology(NTT) papermaking machine.
 13. The papermaking machine of claim 1,wherein the papermaking machine is a Quality Rush Transfer (QRT)papermaking machine.
 14. The papermaking machine of claim 1, wherein thepapermaking machine is an Energy Efficient Technologically AdvancedDrying (ETAD) papermaking machine.